Process for producing adherent coatings on stainless steel to facilitate drawing operations



Patented May 1, 1951 PROCESS FOR PRODUCING ADHERENT COATINGS N STAINLESS STEEL TO FACILITATE DRAWING OPERATIONS Paul Amundsen, Detroit, Mich., and John A. Henricks, Lakewood, Ohio, assignors, by mesne assignments, to Parker Rust Proof Company, Detroit, Mich., a corporation of Michigan No Drawing. Application September 4, 1948, Serial No. 47,938

11 Claims. (01. 14s 's.24)

I The present invention relates to a novel process for producing adherent coatings on stainless steel to facilitate drawing operations. By the term :stainless steels we mean steels which.

contain elements such as chromium with or without nickel, to prevent tarnish of the metal on exposure. Working of such steels in' dies, particularly cold drawing in dies has been very difficult because of the nature of the surfaces of such stainless steels. a

I In cold drawing of carbon steels .it has been found that drawing operations are facilitated by providing an adherent phosphate coating on the, metal prior to the drawing operations. At-

tempts to produce a similar coating on stainless steel surfaces have not produced a coating hav ing satisfactory properties of adherence, which at the same time will facilitate the cold drawing of the stainless steel material and thus facilitate the working thereof in drawing dies.

It is therefore, an important object of the present invention to provide a novel process for producing an adherent coating on stainless steel by chemical means, the process being readily adapted for use on an economically feasible basis in commercial scale operations- It is a further object of the present invention to provide a novel process by which a chemically produced adherent coating is provided on stainless steel surfaces in a relatively short time, the produced coating being eifective to improve the drawing properties of the stainless steel thus facilitating its cold drawing in manufacturing operations, increasing the life of the drawing dies and equipment and permitting the making of deeper draws in a single operation than has heretofore been possible in stainless steel drawing operations,

Other objects of the invention will appear in the following description and appended claims.

Before explaining the present invention in detail,

certain conditions, very well suited for use in I forming coatings on stainless steels in view of their known reactions therewith. It has been suggested in German Patent 731,045 that oxalic acid .may be used with sulfides .or hydrogen sulfide (H28). .While: these solutions will 10.1fm

coatings on the lower stainless steel alloys, the reaction requires a long time at high temperature. cially the ones containing molybdenum, do not react to form any coating. As is known, hydrogen sulfide (HzS) is very poisonous and obnoxions and the difficulty in handling this material has made the process very impractical.

We have found that certain sulfur compounds which contain oxygen and. which yield sulfur dioxide (S02) and sulfur, can be used in the presence of oxalic acid and a soluble halide to form excellent coatings on stainless steel surfaces in a time interval of from about 2 to 10 minutes. Satisfactory compounds include thiosulfates, hydrosulfites, tetra-thionates and -'sul-" fites, together with alkali or ammonium halides such for example as sodium fluoride or bifluoride and/or sodium chloride.

' The term alkali in'the specification and appended claims is to include both the alkali metals and ammonium. The fluorides especially have proven to be effective and are preferred when high alloyed steels, containing molybdenum are to be coated.

Using such materials, a satisfactory coating is formed in 2 to 3 minutes at room temperature on 18-8 stainless steel. molybdenum the temperature may be raised to 14 0-l5 0 F. in which event the time interval may be lengthened to '7 minutes.

The produced coatings are unlike coatings previously produced. On visual inspection theyare dark gray in color. They are very adherent and form an ideal base for an additional conventional drawing compound or lubricant. For stainless steeltube drawing, a chlorinated wax emulsion has proven to be successful, whereas for stainless steel headlight rims, a simple soap solution such as sodium or potassium oleate in a dilute aqueous medium is satisfactory.

The concentration of the oxalic acid is not (Approx) 5% oxalic acid, dihydrate (Approx) 2% ammonium bi-fluoride (Approx) 0.2 %-0.5% sodiumthiosulfate Water to make Some higher stainless steel alloys, espe-' If the alloy contains oxalic acid sodium chloride (Approx;) 1 1% sodium ;bi-fluoride (Approx) 2% sodium thiosulfate Water to make 100% Such a solution is heated to reactive temperatures, usually within the rang'eof from approxi mately room temperature to approximately 190 F. and the properly cleaned stainlesssteel surfaces are immersed therein until a substantial coating is formed therein. This usually requires a time interval of from approximately 2 to approximately 10 minutes. The surfaces will be coated under these conditions and are then water rinsed and either dried or directly im.-' mersed in the drawing compound and subjected to the draw. In cleaning such surfaces they preferably are either alkali cleaned or pickled and water rinsed. I

'Ihe.- solution as described above is tested for strength with N/lO sodium hydroxide, using a 5 ml. sample and methyl orange and phenolphthalein respectively. Such a solution tests free acid 23, total acid 34.5, ratio 1:1.6. This acid ratio slowlytends to increase to 1:280 and even higher. If it gets above 1:2.4 the coating action becomes sluggish, and this can be remedied by removing'pa-rt ofthe sodium salts which crystallize out when-the solution is cold, or by adding hydrochloric acid (HCll or hydrofluoric acid (.I-IF) 1 -While we do not; at this time, fully understand .allof the reactions which occur in such a process, it-is our present belief that the soluble halide has an activator effect which overcomes the normal passivity of the stainless alloys and allows the coating-reactionto be carried out much more rapidly and at much lower temperatures than-in comparable processes in which the solublehalide is not used.

- We do know however, that the use of the above activators for oxalic acid, causes the precipitation of an extremely smooth and adherent film of sulfide to be deposited on the surface, over which the normal coating of ferrous and other metal oxalates is formed. The sulfide containing film isin itself a lubricant and the more adsorptive normal type of oxalate which is formed, is heavier and seems to serve as an ideal base for the'lubricant. It is possible to remove thetop layer of the coating by polishing the surface, and the darkthin film of metallic sulfide is clearly visible. y

i We consider the ftuiction of the sulfur compound used to be that of a hydrogen depolarizer since hydrogen evolution is minimized, although may be a' matter of conjecture. Under certain conditions these compounds may be considered reducing agents-but we believe that their action here is more of an oxidizing nature.

Asa class, we have found that the soluble halides are satisfactory activators and that of this group, thepresently preferred soluble halides comprise the" soluble alkali and ammonium salts of the fluorides-and chlorides.

I. Having thus described the invention, what we desired to secure by Letters Patent of the United States is: i

We claim:

1. In a process for-treating the surfaces of stainless steel to facilitate cold drawing byproducing an adherent coating thereon, the novel step of immersing the stainless steel in an (Approx) 5% (Approx) 2% 4 aqueous bath consisting essentially of oxalic acid, a sulfur compound containing oxygen which yields sulfur dioxide and sulfur under the conditions of the coating bath, and an activator consisting of a soluble alkali halide, fora time interval sufficient to form a substantial coating and at a reactive temperature withinthe range of from approximately room temperature to approximately F;

2; In a process for treating the surfaces of stainless steel to produce an adherent coating thereon as claimed in claim 1 and further characterized in that the oxygen containing sulfur compound is selected from the group consisting of thiosulfates', hydrosulfites, tetra-thionates and sulfites.

3. In a process for treating the surfaces of stainless steel to produce an adherent coating thereon as claimed in claim 1 and further characterized in that the soluble halide is selected from the group consisting of the soluble alkali andammonium salts of the chlorides o1 fluorides.

4. In a process for treating the surfaces of stainless steel to produce an adherent coating thereon as claimed in claim 3 and further char-- acterized in that the soluble halide is an alkali fluoride.

- 5. ma process for treating the surfaces of stainless steel to produce an adherent coating thereon as claimed in claim 3 and further characterized in that the oxygen containing compound is selected from the roup consisting of thios'ulfates, hydrosulfites, tetra-thionate's and sulfites.

6. A bath adapted to produce a coating on stainless steel surface and comprising an aqueous solution containing approximately 5% oxalic acid ('dihydrate), approximately 2% am-' monium bi-fluoride, 0.2% sodium thiosulfate and the balance water.

'7. A bath adapted to produce a coating on a stainless steel surface and comprising an aqueous solution containing approximately 5% o'xalic'aci'd- (dihydrate), 2% sodium chloride, 1% sodium bi-fluoride and 0.2% sodium thiosulfate and the balance water.

8. A bath adapted to produce a coating on stainless steel surfaces and comprising an aqueous solution containing approximately 5% oxalic acid (dihydrate), sodium thiosulfate in an amount ranging from approximately 0.2% to 0.5%, an activator selected from the group consisting of ammonium and sodiumbifiuoride in an amount of from approximately 1% to approximately 2%, and the balance water.

9; In a process for treating the surfaces of stainless steel to produce an adherent coating thereon the novel step of immersing the stainless steel in a bath containing approximately 5% oxalic acid (dihydrate), approximately 2% ammonium bi-fiuoride, 0.2% sodium thiosulfate and the balance water for a time interval sufficient to form a substantial coating .at a temperature within the range of from approximately room temperature to approximately 190 F.

v 10. In a process for treating the surfaces of stainless steel to produce an adherent coating thereon the novel step of immersing the stainless steel in a bath comprising approximately 5% in the range of from approximately room tern-' perature toapproximately 190 F.

11. In a process for treating the surfaces of REFERENCES CITED stainless Steel to produce an adherent coating The following references are of record in the thereon the novel step of immersing the stainfile of this patent: less steel in a bath containing approximately 5% oxalic acid (dihydrate), sodium thiosulfate in an 5 UNITED STATES PATENTS amount ranging from approximate1y1.-O.2% to Number Name Date 0.5%, an activator selected from. the group of 1,911,537 Tanrfer May 30, 1933 ammonium and sodium bifluoride in-an amount 3 of from approximately 1% to 2% and th e balance FOREIGN PATENTS water, for a time interval sufficient to foifm a subm Number o y Da gstantial coating at a temperature within the 554,280 Germany y 932 :range of from approximately room temperature 7 1,04 Germany Feb. 1, 1943 u to approximately 190 F.

PAUL AMUNDSEN. JOHN A. HENRICKS. 15 

1. IN A PROCESS FOR TREATING THE SURFACES OF STAINLESS STEEL TO FACILITATE COLD DRAWING BY PRODUCING AN ADHERENT COATING THEREON, THE NOVEL STEP OF IMMERSING THE STAINLESS STEEL IN AN AQUEOUS BATH CONSISTING ESSENTIALLY OF OXALIC ACID, A SULFUR COMPOUND CONTAINING OZYGEN WHICH YIELDS SULFUR DIOXIDE AND SULFUR UNDER THE CONDITIONS OF THE COATING BATH, AND AN ACTIVATOR CONSISTING OF A SOLUBLE ALKALI HALIDE, FOR A TIME INTERVAL SUFFICIENT TO FORM A SUBSTANTIAL COATING AND AT A REACTIVE TEMPERATURE WITHIN THE RANGE OF FROM APPROXIMATELY ROOM TEMPERATURE TO APPROXIMATELY 190* F. 